1. Field of the Invention
The present invention relates to a power module for motor drive control and other applications which involve the control of a relatively large current.
2. Description of the Related Art
A description is given of a power module which is used, for example, for motor drive control in an automotive power steering system. FIG. 7 shows the outline of a control circuit of the motor. Connected to motor M is an H bridge circuit which includes four switches 5A through 5D comprised of MOS-FETs. For instance, if switches 5A and 5D are turned ON and a current is allowed to flow in the direction of arrow A shown by a solid line, then motor M rotates in the forward direction; if switches 5B and 5C are turned ON, then the current flows in the direction of arrow B shown by a dashed line, causing motor M to rotate in the reverse direction. The rotational speed of the motor M is controlled by changing, for example, the duty ratios of the control pulses supplied to the switches (PWM), thereby changing the current flowing through motor M. Reference numeral 3A denotes a resistor for detecting overcurrent. A microcomputer 50 performs the control of the motor M via an interface 60, the control including primarily the supply of control pulses to the switches 5A through 5D and the detection of an overcurrent. The microcomputer 50 and the interface 60 both operate on a low-voltage circuit of 5 V (volts) with a current in the order of mA (milliampere), while a large current ranging, for example, from 50 to 75 A (amperes) flows through the H bridge circuit which includes motor M, switches 5A through 5D, and resistor 3A for detecting overcurrent. Switches 5A through 5D control a large current in accordance with a low-voltage control pulse supplied from the microcomputer 50 via the interface 60.
FIG. 8 shows a cross-sectional view of conventional power module. In the drawing, numeral 100 denotes a power module, numeral 1 a heat radiating container made of aluminum or copper, numeral 1a a serrated section for improving heat radiation by increasing the surface area, and numeral 2 a metallic substrate which is constituted by an aluminum plate 2a on which a resinous insulating layer 2b made of epoxy resin or the like, and which is approximately 15 to 30 .mu.m thick is formed. Numeral 2c denotes a pin which fixes the metallic substrate 2 to the heat radiating container 1, and numeral 2d denotes silicone grease for improving the heat conduction between the metallic substrate 2 and the heat radiating container 1. Numeral 3 indicates a resistor such as a shunt resistor for detecting overcurrent, numeral 4 a copper heat sink, numeral 5 a MOS-FET which is a semiconductor device which constitutes the switch of FIG. 7, numeral 6 an aluminum wire which connects the MOS-FET 5 and the circuit pattern on the metallic substrate 2, numeral 7 a lead wire which is set on the circuit pattern 21 of the metallic substrate 2 and which is used for external connection, numeral 8 an insulated circuit board which is made of glass epoxy resin or the like and on which the control circuit for the MOS-FET 5 is mounted, numeral 8a a control package which incorporates the microcomputer 50, the interface 60, etc. of FIG. 7, numeral 9 a lead wire which electrically connects the metallic substrate 2 and the circuit board 8, and numeral 11 a silicone gel which is a protective material (coating resin) for moisture proofing effect. Although FIG. 8 shows only one MOS-FET 5, at least four MOS-FETs 5 are required as shown in the circuit of FIG. 7, and the MOS-FETs 5 are mounted on the metallic substrate 2. In actual use, however, four or more MOS-FETs are normally mounted because the switches 5A and 5B shown in FIG. 7 must be constituted by connecting a plurality of MOS-FETs in parallel to allow a smaller current to flow through each of the MOS-FETs 5 and minimize the ON resistance, thus dispersing and reducing the heat generated in the MOS-FETs 5. Likewise, a plurality of resistors 3 are mounted thereon. The circuit board 8 is fixed to the heat radiating container 1 with an adhesive agent or the like which is not shown.
A large current flows into the H bridge circuit which comprises the MOS-FETs 5 and the resistors 3 for detecting overcurrent, and which is designed to drive and rotate the motor in the forward and reverse directions, accordingly, the H bridge circuit generates a great deal of heat. For this reason, the H bridge circuit is mounted on the metallic substrate 2 which features high heat conductivity and good heat radiation. On the other hand, the circuit board 8 with a circuit mounted thereon through which a minute current for controlling the operation of the MOS-FETs 5 flows does not require heat radiation; therefore, it is installed in a position isolated from the metallic substrate 2 so as to be free from the influences exerted by heat radiated from the metallic substrate 2.
A brief description is given of the connection between the circuit pattern 21, which is made of copper or the like and which is mounted on the insulating layer 2b of the metallic substrate 2, and other parts. An aluminum layer is formed on the circuit pattern 21 to which the aluminum wire 6 is connected, the aluminum wire 6 being connected and fixed to the aluminum layer by aluminum wire bonding. The resistor 3, the copper heat sink 4, and the lead wire 7 for external connection are connected and fixed onto the circuit pattern 2t by soldering. The MOS-FETs 5 are also fixed onto the copper heat sink 4 by soldering. The circuit pattern 21 on the metallic substrate 2 measures about 100 .mu.m thick and 7 mm wide to allow a current of 50A to 75A to flow through and it connects the MOS-FETs 5 with the elements such as the resistors 3.
In the conventional power module configured as described above, it is necessary to provide the surface of the metallic substrate, on which the heat radiating MOS-FETs and resistors are mounted, with a resinous insulating layer to form the circuit pattern 21 for connecting the heat radiating components on the metallic substrate. Such a resinous layer, however, has extremely low heat conductivity, resulting in poor heat radiation of the whole heat radiation passage from the metallic substrate 2 and the aforesaid heat radiating elements to the heat radiating container. Accordingly, as described above, in order to minimize the current flowing through the MOS-FETs and the ON resistance, the plurality of MOS-FETs are connected in parallel to constitute a single switch, thus controlling the heat generated in the MOS-FETs; however, increasing the number of devices led to lower wiring efficiency and a larger board.
To cope with the problem, above described a direct bond copper (DBC) substrate or an aluminum nitride (AlN) substrate is utilized instead of the metallic substrate 2, the DBC substrate and the AlN substrate being made of an alumina (Al.sub.2 O.sub.3) plate and an aluminum nitride (AlN) plate, respectively, provided with a nickel (Ni) layer and a copper (Cu) layer on the surfaces of the plates, respectively, to metallize the plates. Then the metallized substrate is soldered to the heat radiating container which has a plated surface. This, however, posed the following problem.
The DBC substrate or the AlN substrate described above is electrically insulated and it exhibits better heat radiation than the metallic substrate; however, the alumina and the aluminum nitride are castings and are prone to break and are mechanically weak at the electrodes thereof, in particular, the portions where the external connection lead wire 7 is installed. In addition, this type of substrate has a considerable difference in the coefficient of thermal expansion between the alumina plate or the aluminum nitride plate, which is a casting, and the surface nickel or copper layer which is metallic. Hence, making the substrate larger unavoidably increases the thermal stress (the larger the substrate, the larger the warp). As a result, the substrate tends to break when it is subjected to the heat from soldering it to the heat radiating container or the heat generated by switching many MOS-FETs (thermal fatigue).
There is another problem associated with this design. The aluminum wire needs to be covered with silicone gel for moisture proof effect; in the case of the conventional power module having the structure explained above, the whole cavity in the heat radiating container 1, where the metallic substrate 2 is disposed, must be filled with silicone gel, requiring a large quantity of silicone gel. Thus, the conventional power module had the problems described above.